Method for treating a photovoltaic active layer and organic photovoltaic element

ABSTRACT

The invention relates to an organically based photovoltaic element, in particular a solar cell comprising a photovoltaically active layer whose absorption maximum can be shifted into the longer wavelength region and/or whose efficiency can be increased.

The invention relates to an organically based photovoltaic element,particularly a solar cell comprising a photovoltaically active layerabsorbing in the blue region.

Organically based solar cells are known from U.S. Pat. No. 5,331,183 of1994 and numerous subsequent publications.

Known in particular are organic solar cells based on polyalkylthiophene(P3AT). A typical cell structure for this photovoltaic element includesthe following layers: an anode, composed, for example of ITO (indium tinoxide), overlain by a hole-conducting layer of a copolymer such as amixture of PEDOT with PSS as the anion. Topping that is a layer ofP3AT:PCBM [poly(3-hexylthiophene) mixed with phenylC₆₁-butoxymethoxy],which is the photovoltaically active layer. Over that is the cathodelayer, composed for example of a metal such as aluminum or a Ca/Agalloy. The individual layers can differ from this scheme, however; inparticular, both the electrodes and the acceptor (PCBM) can be made ofanother material. Cyano-substituted PPVs (CN PPVs), for example, havealready been used as acceptors; but arbitrarily many additions to thepolythiophene can be contemplated.

There is a need to shift the absorption maxima of the photovoltaicallyactive layer into the longer wavelengths, since, for one thing, mixingpolythiophene with fullerene causes a blue shift of the absorptionmaximum. This increases the mismatch, i.e., discrepancy, between theabsorption maximum and the peak emission of sunlight.

An object of the invention is to provide a method by which theabsorption maximum of a photovoltaically active layer can be shiftedinto the longer wavelength region and/or its efficiency improved (e.g.by increasing the short-circuit current). It is in particular an objectof the present invention to provide a method by which the absorptionmaximum of a photovoltaically active layer containing apoly(alkyl)thiophene in mixture with a fullerene can be shifted into thelonger wavelengths.

The invention is directed to a method for treating a photovoltaicallyactive layer with a solvent and/or by annealing, characterized in thatthe photovoltaically active layer comes into contact with solvent,molecules and/or is heated. The invention is also directed to aphotovoltaic element that comprises a photovoltaically active layercontaining polyalkylthiophene in mixture and that absorbs in the deepred region.

The photovoltaically active layer is preferably a polyalkylthiophenethat is present in mixture with an additive such as a fullerene,particularly a methanofullerene. Further possible additives instead ofthe fullerene would be, for example, inorganic nanoparticles based onCdTe (cadmium telluride), CdS (cadmium sulfide), polymers having a highelectron affinity, such as, for example, cyano-substituted PPVs (CNPPVs) or small molecules having a high electron affinity, such as, forexample, tetracyanoquinone (TCNQ) or tetracyanoanthraquinodimethane(TCAQ).

In one embodiment of the invention, the photovoltaically active layer isexposed to a solvent vapor at room temperature. This can be done, forexample, by passing (holding) the photovoltaically active layer over avessel containing solvent and/or conducting the solvent vapor over thephotovoltaically active layer.

In one embodiment, the photovoltaically active layer is exposed to thesolvent vapor only very briefly, i.e., for less than one minute or, forexample, in only the second or millisecond range [syntax sic].

In one embodiment of the invention, the photovoltaically active layer isannealed at a temperature of at least 70° C., preferably about 80° C. orhigher. The progress of the annealing can be monitored via the increasein the short-circuit current. Other temperature and time combinationsare conceivable; the process is assumed to be completed as soon as thephotovoltaic parameters cease to improve. The annealing can be performedby placing the photovoltaically active layer in a drying oven or on ahot plate or the like. The solvent treatment can also take place at thesame time as the annealing.

The solvents used can, for example, be aromatic solvents such as xylene,toluene or the like, or halogen-containing solvents such as chloroformor the like. Choice of the right solvent depends on the mixture of thematerial forming the photovoltaically active layer. The effect of thesolvent is, for example, that the solvents xylene, toluene, butanoneand/or chloroform and/or a further solvent or an arbitrary mixture ofsaid solvents at least partially etch and/or soften polyalkylthiophene.

The photovoltaically active layer is produced in a conventional manner;according to the state of the art, for example, a spin-coated film isformed from a P3AT [poly(3-alkylthiophene)]/PCBM(phenylC₆₁-butoxymethoxy) solution or applied by standard printingmethods (silk screening, flexography, etc.).

The figure is explained more specifically hereinbelow on the basis ofthree graphs reflecting test results.

FIG. 1 illustrates the observed effect of solvent vapors on theabsorption of P3AT films spin-coated from chloroform, with and withoutfullerene, on glass. The triangles signify a pure P3AT film on glass andthe solid squares a P3AT/PCBM film. It is clearly apparent that thisfilm lacks the absorption contribution in the wavelength range around550 nm that is typical of P3AT. Once the film has been exposed tochloroform vapor (open diamonds), its absorption behavior changes andthe absorption characteristics typical of P3AT are again in evidence.

FIG. 2: variation of short-circuit current ISC (solid squares) andefficiency (solid circles) with the temperature at which the layer wasannealed. Each specimen (structure: ITO/PEDOT/P3HT:PCBM/Ca/Ag) wasannealed for 20 minutes and its electrical characteristics (Isc andefficiency) were measured at room temperature under illumination with 70mW/cm² white light from a xenon lamp. It can be seen that theshort-circuit current, and therefore the efficiency, begin to increaseat a temperature of >80° C.

FIG. 3: current/voltage (I/V) characteristic of cells undergoing onetemperature treatment before (solid circles) or after (solid squares)solvent vapor treatment. The increase in short-circuit current (Isc) andefficiency reflects the red shift in the absorption behavior of the cell(as illustrated in FIG. 1).

Mixing P3ATs, especially polyhexylthiophenes, with fullerenes causes theabsorption maximum of the P3AT to shift more than 100 nm into the bluespectral region. This increases the spectral mismatch between the solarcell and the sun's spectrum. The invention solves the followingproblems:

a.) shifting the absorption of P3AT/fullerene films back into the redspectral region by solvent annealing and

b.) increasing the efficiency of the solar cell by temperatureannealing.

“Annealing” denotes the treatment of a photovoltaically active layer inthe context of this invention in order to achieve the object, i.e., tobring about a red shift in the absorption maximum of the layer.

1. A method for treating a photovoltaically active layer with a solventand/or by annealing, characterized in that said photovoltaically activelayer comes into contact with solvent molecules and/or is heated.
 2. Themethod as defined in claim 1, wherein said photovoltaically active layeris a polyalkylthiophene that is present in mixture with an additive suchas a fullerene, particularly a methanofullerene.
 3. The method asdefined in either of claim 1, wherein said photovoltaically active layeris exposed to a solvent vapor.
 4. The method as defined in claim 3,wherein said photovoltaically active layer is exposed to said solventvapor at room temperature.
 5. The method as defined in claim 1, whereinsaid photovoltaically active layer is exposed to said solvent vapor forno longer than one minute.
 6. The method as defined in claim 1, whereinsaid solvent xylene, toluene, butanone and/or chloroform and/or afurther solvent and/or an arbitrary mixture of said solvents at leastpartially etches or softens said polyalkylthiophene.
 7. The method asdefined in claim 1, wherein said photovoltaically active layer isannealed at a temperature of at least 70° C.
 8. A photovoltaic elementcomprising a photovoltaically active layer containing apolyalkylthiophene in mixture, wherein the photovoltaically active layerhas an absorption maximum in the deep red region.
 9. A method oftreating a photovoltaically active layer, comprising: contacting thephotovoltaically active layer with solvent molecules.
 10. The method asdefined in claim 9, wherein the photovoltaically active layer comprises:a polyalkylthiophene; and a fullerene mixed with the polyalkylthiophene.11. The method of claim 10, wherein the fullerene comprises amethanofullerene.
 12. The method of claim 9, wherein the solventcomprises solvent vapor.
 13. The method of claim 11, wherein the solventvapor is at room temperature.
 14. The method of claim 11, wherein thephotovoltaically active layer contacts the solvent vapor for no longerthan one minute.
 15. The method of claim 9, wherein the solventcomprises at least one solvent selected from the group consisting ofxylene, toluene, butanone, and chloroform.
 16. The method of claim 9,wherein the solvent at least partially etches or softens thepolyalkylthiophene.
 17. The method of claim 9, further comprisingannealing the photovoltaically active layer.
 18. The method of claim 17,wherein the photovoltaically active layer is annealed at a temperatureof at least 70° C.
 19. The method of claim 9, wherein, after treating,the photovoltaically active layer has an absorption maximum in the deepred region.
 20. A method of treating a photovoltaically active layer,comprising: heating the photovoltaically active layer at a temperatureof at least 70° C.
 21. The method of claim 20, wherein, after treating,the photovoltaically active layer has an absorption maximum in the deepred region.
 22. The method of claim 1, wherein, after treating, saidphotovoltaically active layer has an absorption maximum in the deep redregion.